Charged particle sources are used for various surface modification, etching and deposition applications, and are particularly advantageous compared to other methods for providing direct control of particle energy and flux, angle of incidence to the substrate, and isolation of the substrate from the conditions of the reactor used to generate the etching and or depositing species.
Broad-beam ion sources, in particular, have numerous applications in microelectronics device fabrication. Ion beam equipment is already extensively used, for example, in the production of high frequency microwave integrated circuits and thin magnetic heads.
In surface modification or ion beam etching, generally known as "ion milling", a beam of ions is extracted from a plasma ion source by electrostatic methods and is used to remove material from a substrate mounted in the path of the beam. In reactive ion milling methods, certain chemical(s) are introduced to the ion source or to the etching chamber which cause chemical reactions to occur on the substrate as part of the milling process. Often the chemical process is affected by energetic assistance by the plasma (in the ion source) and/or the ion beam. An example is the addition of "inert" tetrafluoromethane gas to the ion source, which is broken up into various reactive fluorinated species that increase the rate of etching of certain substrates, such as aluminum oxide or silicon dioxide.
There are two basic configurations for ion beam deposition. In "primary" or "direct" ion beam deposition, an ion beam source is used to produce a flux of particles, including constituents of the desired film, which are accumulated at the substrate. In one category of "primary" ion beam deposition, the deposited material is formed by reactive means from precursor chemicals introduced to the ion source, usually in the gas phase. An example of great practical value is diamondlike carbon films formed from direct ion beam deposition from an ion source operated on hydrocarbon gas(es), such as methane.
The other general configuration in which ion beams can be used for thin film deposition is commonly known as "secondary ion beam deposition", or "ion beam sputtering". In this method, an ion beam consisting of particles that are not essential to the deposited film are directed at a target of the desired material, and the sputtered target material is collected on the substrate. Secondary ion beam deposition can be a completely inert sputtering process. Alternatively, certain chemicals can be added to the ion source or elsewhere in the deposition chamber to alter the chemical properties of the deposited film either by reaction with the target material or with the substrate. This can be done with or without energetic activation by the ion source plasma or the ion beam.
Other types of charged particle sources include electron sources and negative ion sources. Electron beams can be used in industrial applications for property or reactive modification of thin films. Electron beams are distinguished from ion beams in that the electrons have almost no momentum, and thereby are less disruptive to the surface of the substrate. Negative ion sources have been developed for research application. In particular, beams of negative hydrogen ions are of interest for possible use in fusion energy sources.
In a typical charged particle source (or gun) electron bombardment of neutral gas atoms or molecules in a contained vessel is employed to create a plasma from which the desired charged particle species is extracted by an appropriate means. A continuous, stable, efficient and practical particle beam source typically comprises the following basic components: (1) a mechanism to provide an uninterrupted supply of fresh neutral gas species; (2) an energizing device to ensure constant supply of high energy electrons for ionization; (3) a facility for continuous removal of spent gas species and control of the operating pressure by a high vacuum pumping system, which is located in the process chamber on which the particle gun is mounted; (4) a mechanism of controlling the energy of the particle beam with respect to the target at which it is aimed, through control of the plasma potential with respect to ground; (5) a device for enabling the extraction of the desired particles through an opening in the charged particle source while simultaneously preventing particles of the opposite charge from leaving the charged particle source through the same opening (the particle optics); and (6) a mechanism to electrically compensate the plasma for the extraction of charged particles of one polarity in order to maintain its quasi-neutrality (to prevent charging of the charged particle source and subsequent instability).
In practice, components (4) and (6) are the same. That is, the electrode and power supply which controls the plasma potential with respect to ground by charging the plasma also maintains the plasma stable at that level by providing a path for charge compensation. This electrode shall be referred to herein generally as the "plasma potential control electrode." For example, in an ion source, the ion current which is extracted from the ion source is compensated by an equal current of electrons extracted from the plasma to ground through the plasma potential control electrode, which is connected to a positive high voltage beam supply. For a source of positively charged particles, the plasma potential control electrode is referred to as the "anode."
In a typical source, the ionizing electrons are produced from a cathode which is connected to the negative terminal of a discharge power supply, the positive terminal of which is connected to an anode which is in contact with the plasma. The energy of the electrons is controlled by the voltage of the discharge power supply. For example, in order to efficiently ionize Ar ions, the discharge voltage should be greater than 15 eV and is typically set between 20 eV and 60 eV. The plasma and the entire discharge power supply is electrically isolated from ground and floated to the desired plasma potential by connection with the beam power supply. This connection is usually made to the discharge cathode or anode described previously. For example, to provide an ion beam of singly charged Argon ions with a desired energy of about 500 eV, the positive terminal of the beam supply is connected to the discharge anode and set to 500 V. The cathode is usually a heated filament or hollow-cathode, but may also be a cold cathode emission. As a second example, to provide a 1 keV electron beam, the negative terminal of the beam supply is connected to the discharge anode and set to 1000 V. Charged particle sources which use the above described methods of plasma generation are categorized as "DC" sources.
An early version of an industrial DC source is described in U.S. Pat. No. 3,913,320 issued in 1975 to Reader and Kaufman. This type of ion source was developed originally for space propulsion. Various modifications of the Kaufman source have since been disclosed, which are primarily designed to optimize the efficiency of the source and to improve the method of extracting the ions or shaping the beam profile for ion beam etching and deposition applications. See for example U.S. Pat. No. 4,873,467 issued in 1989 to Kaufman. The above described sources have in common the use of a heated cathode, either a heated filament or hollow cathode. A cold cathode electron emitter which may be used as an ionization source in the chamber of an ion gun is described in U.S. Pat. No. 4,739,214 issued in 1988 to Barr. A cold cathode plasma anode electron gun is described in U.S. Pat. No. 4,707,637. U.S. Pat. No. 4,684,848 discloses a broad beam electron source. Various ion sources designs including negative ion sources (e.g. p. 299-309) are discussed in the Handbook of Ion Sources, ed. by B. Wolf, published in 1995 by CRC Press.
DC sources have disadvantages compared with other sources for etching and thin film deposition techniques in terms of charged particle source maintenance and reactive gas compatibility. Charged particle sources with filament type cathodes, for example, are the easiest to operate and maintain, but require frequent replacement of the filament assembly. Furthermore the hot filaments are rapidly attacked in the plasma state by gases which are useful for thin film deposition and etching, such as hydrocarbons, oxygen, hydrogen, and fluorinated gases. Charged particle sources equipped with hollow cathodes are difficult to maintain. They also cannot be operated with high concentrations of reactive gas because the hollow cathodes are easily contaminated and must be protected by continuous purging with inert gas. Cold cathodes can be readily maintained and are compatible with some reactive gases (e.g. oxygen) but have other limitations, such as generally low particle beam density, and poor beam collimation. These shortcomings of DC sources hinder the implementation of ion beam processes in manufacturing processes.
We have found that the above-mentioned disadvantages can be avoided by using radio frequency (RF) charged particle sources which employ high frequency electromagnetic energy for ion generation, including microwave energy sources. An optimally designed RF charged particle source has the following general attributes:
applicability for reactive gases like oxygen, halogen components, etc. due to absence of discharge filaments; PA1 simple and rugged design easy to assemble and dissemble modest power supply and control requirements, easy ignitability; PA1 discharge stability, reliable fault-free and long duration operation; PA1 reduced concern for contamination of substrates due to reduced sputtering of the source components and materials and optimized material design (e.g. quartz instead of stainless steel chamber).
RF Inductively coupled ion sources were originally developed for space propulsion starting in 1960. See "State of the Art of the RIT-Ion Thrusters and Their Spin-Offs" by H. Loeb, et. al. of Giessen University (1988) which describes an ion source with an axial RF coil. An inductively coupled RF ion source with a flat RF coil design is disclosed in U.S. Pat. No. 5,198,718 granted to Davis, et. al., in 1993. An ion source with an internal RF coil is shown on p. 104 of Wolf's Handbook. RF capacitively coupled ion sources, such as the one shown on p. 230 of Wolf's Handbook, and U.S. Pat. No. 5,274,306 issued 12/93 to Kaufman are also known. An electron cyclotron resonance ion source is described by Ghanbari in U.S. Pat. No. 4,778,561 issued in 1988.
In contrast with DC sources, many RF sources do not require any discharge electrodes directly in contact with the plasma. However as mentioned above, an electrode must be provided to control the plasma potential and provide for charge compensation of the plasma. This may be combined with some other function. For example, in Loeb (1988) this function is performed by the gas distributor. In U.S. Pat. No. 5,198,718 it is performed by the "screen" grid portion of the ion optics.
One general limitation of prior art charged particle sources in practical applications is the formation of high electrical resistivity precipitates or films on electrode surfaces as a result of decomposition of certain gases or from physical sputtering of other dielectric materials. Such dielectric materials may, for example, include the walls of the plasma vessel, which are often constructed of quartz in RF and microwave plasma sources. In general, plasma and radical concentrations are strongly sensitive to reactor surface conditions. Changes in the conductivity of the electrode surfaces can lead to problems of aging and irreproducibility and can cause charge buildup in the ion source by inhibiting current flow between the plasma and the electrode which is used to control the plasma potential.
Stability improvement can be achieved by special source conditioning procedures. However, the problems of aging and irreproducibility become more complicated if conditioning of the source internal surfaces and source operation is accompanied with deposition on the walls and electrodes of high electrical resistivity precipitates.
In practical applications there are many gases such as hydrocarbon, halocarbon gases, etc. that react inside the charged particle source during operation to form large amounts of high electrical resistivity precipitates. For these cases the above-mentioned limitation greatly hinders the application of charged particle sources for production thin film deposition and etching.
There is a clear need for the broad-beam charged particle source utilizing reactive gases that is capable to prevent accumulation of electrical charge in the source during the source operation.
It is an object of the present invention to provide a stable charged particle source, especially for operation with reactive gases, such as hydrocarbons, halocarbons, etc. that may form high electrical resistivity precipitates inside of the source.